1. Field of the Invention
The present invention relates to a magnetooptical recording/reproducing apparatus provided with first and second bias magnetic field generation devices, wherein a laser beam is irradiated to a recording medium from an optical head movable with respect to the recording medium to form a light spot on the recording medium, and the first and second bias magnetic field generation devices are properly used according to plural modulation schemes to effect recording of information by applying a magnetic field to a very small region on which the light spot is formed.
2. Related Background Art
As one conventional magnetooptical disk apparatus for performing magnetooptical recording/reproduction, an apparatus, which performs recording based on a so-called optical modulation method, i.e., applies a predetermined magnetic field from a bias magnetic field generation device to a recording region of a magnetooptical recording medium, and radiates a laser beam modulated according to recording information from an optical head to the recording region, is known. As another apparatus, an apparatus, which performs recording based on a magnetic field modulation method, i.e., radiates a constant laser beam from an optical head to a recording region, and applies a magnetic field modulated according to recording information from a bias magnetic field generation device to the recording region, is known. Thus, a demand has arisen for a magnetooptical disk apparatus which has compatibility allowing recording/reproduction of information on both magnetooptical recording media of these different methods.
For this purpose, conventionally, as shown in FIGS. 1 to 4, a magnetooptical disk apparatus which has a structure using two different bias magnetic field generation devices has been proposed. In this case, a bias magnetic field generation device A used for recording in the magnetic field modulation method is arranged as a first bias magnetic field generation device, and a bias magnetic field generation device B used for recording in the optical modulation method is arranged as a second bias magnetic field generation device.
As is well known, in the bias magnetic field generation device A, a floating slider 2 which mounts a magnetic head as a bias magnetic field generation unit is supported by a support member 2b via a support spring 2a as a load beam, and the support member 2b is pivotally supported, via a pivot shaft 12, on a carriage 10 which carries an optical system. A torsion coil spring (not shown) is arranged around the pivot shaft 12 so as to bias the floating slider 2 against a magnetooptical disk 40. The carriage 10 is movable in the radial direction (X direction in FIG. 2) of the disk along guide rails 8. The two ends of each of the guide rails 8 are fixed to a base 9.
The magnetooptical disk 40 contained in a disk cassette 30 is rotatably placed on a spindle motor 80. The spindle motor 80 is supported by an apparatus main body to be movable in the vertical direction (Y direction in FIG. 2). When the disk cassette 30 is not loaded in the apparatus main body, the spindle motor 80 is retracted to a lower portion to avoid interference with the disk cassette 30 to be inserted into the apparatus main body. When the disk cassette 30 is completely inserted into the apparatus main body, the spindle motor 80 is moved upward by a spindle motor lift mechanism (not shown), and clamps the magnetooptical disk 40.
Upon insertion of the disk cassette 30 into the apparatus main body, the magnetic head of the bias magnetic field generation device A must also be retracted. For this purpose, the base 9 is provided with a solenoid driving mechanism 53 having a retracting lever 50 as a flip-up mechanism. When a solenoid 54 is not energized, as shown in FIG. 3, the retracting lever 50 is pulled down by the restoring force of a compression spring 55 to push down a roll-shaped engaging contact portion 7 provided at the rear end of the support member 2b, thereby retracting the magnetic head. When the solenoid 54 is energized, as shown in FIG. 2, since the retracting lever 50 is pulled up, the retracting lever 50 does not contact the engaging contact portion 7, and the floating slider 2 which mounts the magnetic head lands on the disk surface.
The bias magnetic field generation device B is constituted by a coil 4 provided to the carriage 10, and a substantially U-shaped core member 1. Two end portions 1a and 1b of the core member 1 are located on the two sides of the magnetooptical disk 40 to sandwich it therebetween.
FIG. 4 is an enlarged sectional view of a portion near the above-mentioned very small region where the beam spot is formed. Referring to FIG. 4, an objective lens 6 focuses a light beam onto the very small region. An upper yoke 11 is arranged on an upper portion of a driving magnetic circuit (not shown) for the objective lens 6, and is used for preventing a magnetic field from leading the driving magnetic circuit to the very small region. When the end portion 1b of the core member 1 is located near the upper yoke 11, the upper yoke 11 also serves as a portion of a magnetic circuit of the bias magnetic field generation device B.
When the magnetooptical disk 40 is loaded into the apparatus main body, information on a control track of the magnetooptical disk 40 is reproduced to discriminate whether the loaded disk is one for magnetic field modulation recording or for optical modulation recording. If the loaded disk is a magnetic field modulation recording disk, energization to the solenoid 54 is started to bring down the floating slider 2 which mounts the magnetic head onto the disk. If the loaded disk is an optical modulation recording disk, the solenoid 54 is not energized, and the bias magnetic field generation device B is used.
However, in such a magnetooptical disk apparatus, as shown in FIG. 4, the end portion 1a of the core member 1 of the bias magnetic field generation device B must be inevitably arranged at a position offset by a distance d from the very small region where the beam spot is formed, so as to avoid interference with the floating slider 2 of the bias magnetic field generation device A.
In general, such an electromagnet has the highest magnetic flux density at a position right below the core end portion. FIG. 5 is a graph showing a change in magnetic flux density on the very small region where the beam spot is formed when the distance d is changed. At a point A, the generated magnetic flux density assumes a maximum value in the conventional arrangement. A point B is a point where d=0, i.e., corresponds to a case wherein the core end portion of the electromagnet is arranged right above the very small region. As can be seen from this graph, efficiency is very low since the conventional arrangement can use only magnetic field generation performance 1/2 that of the bias magnetic field generation device B. Therefore, the low efficiency has been compensated for by constituting a U-shaped core, increasing the number of turns of the coil, and increasing the sectional area of the core. However, these countermeasures undesirably increase the weight of the bias magnetic field generation device B, and make the optical head heavy, resulting in a slow seek operation.